PMP450 SM fails to connect to AP

I'm hoping someone has run across a solution to an issue we're experiencing with SMs not connecting to their AP.

Briefly: 1. Software v15.0.3 on both APs and SMs

2. Default out-of-the-box SM (except for enabling all frequencies and upgrading software) when aimed to tower will cycle through "Scanning-->Syncing--> trying to register (at 40.213 miles!) even though I'm within 2 miles of the tower. Then the Scanning-Syncing cycle begins all over again endlessly without ever registering to an AP.

3. Other locations will Scan-Sync-Register within about 5 minutes with no problem. I have compared configs and they are set up exactly the same.

4. One sector I tested yesterday connected as expected within about 5 mins. (this is with all frequencies and bandwidths enabled, no color code), but when I defaulted the SM to try it again, it failed to connect the next 3 times, even after clearing idle sessions in the AP. It seems the SM won't lock onto the strongest signal, and in some cases when it does make the connection, it's like a crap shoot--it might connect to a distant AP with 15 dB less signal strength rather than the optimal sector..

5. Seems to be very random, when this will occur.

Has anyone else experienced this behavior?  Thanks!

What frequency band are you using?  Have you done a spectrum analysis at the SM site?

Typically, behavior like this could be a sign of interference in the area.

Yes, I agree that it's acting like that, but spectrum analysis comes out clean. 3.65 - 3.70 GHz. I have four towers in one city that the SMs connect within 3-5 minutes out-of-the-box, then random towers in two other cities that won't let the same SM connect that I tested on the good towers. I'll run another scan today and see if there's been any changes and get back.

Here’s an update to this ticket:
1) Changing the Subscriber Color Code Rescan period made no difference (Cambium Tech Support suggested I try this).
2) Spectrum analysis ran clean.
3) I added the Color Code specific to the AP to which I was attempting to connect—this didn’t help, although once before I had done this and it worked.
4) In the Radio Configuration Tab: I left Color Code at 0, then removed all of the frequencies except for the specific five that we use in our network.
5) I removed all Channel Bandwidths except the 10MHz that we use. (Please see attached jpg)
6) The SM then connected within three minutes to the strongest AP, as it used to. I tested two different sectors with this and it worked on both.
The question remains, however, why four towers in one city work perfectly, even without reducing the number of frequencies to scan from 19 to 5, while other APs will never connect until I reduce the number to 5?
I believe this must have started when we switched to v15.0.3 software. Before that, I don’t remember there being quarter-frequencies included in the frequency list (i.e., 3652.50, 3697.50, etc.).